Improved size-tunable synthesis of monodisperse gold nanorods through the use of aromatic additives

Plasmonic caged gold nanorods for near-infrared light controlled drug delivery

A new near-infrared light-controlled drug delivery system based on caged gold nanorods (CGNRs) is demonstrated. The loading and release process of drug payloads into/from CGNR nanocarriers were systematically investigated. The drug-loaded CGNR constructs could enable combined chemotherapy and photo-thermal effects in killing tumor cells upon light irradiation, therefore, enhance the killing efficiency. In conjunction with visibility under quenching-free dark-field imaging, CGNRs may serve as multifunctional theranostic reagents towards cancer diagnostics and therapeutics.

Hierarchical self-assembly of gold nanoparticles into patterned plasmonic nanostructures

The integration of nanoparticle superstructures into daily life applications faces major challenges including the simplification of the self-assembly process, reduced cost, and scalability. It is, however, often difficult to improve on one aspect without losing on another. We present in this paper a benchtop method that allows patterning a macroscopic substrate with gold nanoparticle supercrystals in a one-step process. The method allows parallelization, and patterned substrates can be made with high-throughput. The self-assembly of a variety of building blocks into crystalline superstructures takes place upon solvent evaporation, and their precise placement over millimeter scale areas is induced by confinement of the colloidal suspension in micron-sized cavities. We mainly focus on gold nanorods and demonstrate their hierarchical organization up to the device scale. The height of the formed nanorod supercrystals can be tuned by simply varying nanorod concentration, so that the topography of the substrate and the resulting optical properties can be readily modulated. The crystalline order of the nanorods results in homogeneous and high electric field enhancements over the assemblies, which is demonstrated by surface-enhanced Raman scattering spectroscopy.

Multidimensional analysis of nanoparticles with highly disperse properties using multiwavelength analytical ultracentrifugation

The worldwide trend in nanoparticle technology toward increasing complexity must be directly linked to more advanced characterization methods of size, shape and related properties, applicable to many different particle systems in science and technology. Available techniques for nanoparticle characterization are predominantly focused on size characterization. However, simultaneous size and shape characterization is still an unresolved major challenge. We demonstrate that analytical ultracentrifugation with a multiwavelength detector is a powerful technique to address multidimensional nanoparticle analysis. Using a high performance optical setup and data acquisition software, information on size, shape anisotropy and optical properties were accessible in one single experiment with unmatched accuracy and resolution. A dynamic rotor speed gradient allowed us to investigate broad distributions on a short time scale and differentiate between gold nanorod species including the precise evaluation of aggregate formation. We report how to distinguish between different species of single-wall carbon nanotubes in just one experiment using the wavelength-dependent sedimentation coefficient distribution without the necessity of time-consuming purification methods. Furthermore, CdTe nanoparticles of different size and optical properties were investigated in a single experiment providing important information on structure-property relations. Thus, multidimensional information on size, density, shape and optical properties of nanoparticulate systems becomes accessible by means of analytical ultracentrifugation equipped with multiwavelength detection.

Gold nanorod translocations and charge measurement through solid-state nanopores

We study translocations of gold nanoparticles and nanorods through silicon nitride nanopores and present a method for determining the surface charge of nanorods from the magnitude of the ionic current change as nanorods pass through the pore. Positively charged nanorods and spherical nanoparticles with average diameters 10 nm and average nanorod lengths between 44 and 65 nm were translocated through 40 nm thick nanopores with diameters between 19 and 27 nm in 1, 10, or 100 mM KCl solutions. Nanorod passage through the nanopores decreases ion current in larger diameter pores, as in the case of typical Coulter counters, but it increases ion current in smaller diameter nanopores, likely because of the interaction of the nanopore's and nanoparticle's double layers. The presented method predicts a surface charge of 26 mC/m(2) for 44 nm long gold nanorods and 18 mC/m(2) for 65 nm long gold nanorods and facilitates future studies of ligand coverage and surface charge effects in anisotropic particles.

Pt@Au nanorods uniformly decorated on pyridyne cycloaddition graphene as a highly effective electrocatalyst for oxygen reduction

Preparing metal-supported graphene nanocomposites is both interesting and challenging because of their well-defined morphologies and have potential application for oxygen reduction reaction (ORR). Here, we present an easy approach to synthesizing a novel hybrid material composed of Pt@Au nanorods (NRs) uniformly dispersed on the pyridyne cycloaddition of graphene (Pt@Au-PyNG), and the material serves as a high-performance catalyst for ORR. This hybrid electrocatalyst significantly decreases the use of Pt by using Pt dispersed on Au NRs and shows a markedly high activity toward ORR. The resulting Pt@Au-PyNG hybrid displayed comparable electrocatalytic activity and better stability than commercial Pt/C in alkaline solutions toward ORR. The hybrid effectively blocks CO formation to increase catalyst resistance to CO poisoning, thereby decreasing the amount of Pt needed. Free-energy diagrams for ORR on Pt@Au (111) through dissociative and associative mechanisms show that OH or O hydrogenation is the rate-limiting step based on DFT calculations.

Diffusion and seed shape: intertwined parameters in the synthesis of branched metal nanostructures

Branched nanocrystals display interesting optical and catalytic properties on account of their high surface areas and tips with small radii of curvatures. However, many synthetic routes toward branched nanocrystals result in inhomogeneous samples on account of asymmetric branching. Seed-mediated coreduction is a recently developed route to symmetrically branched nanocrystals where the symmetry of the seeds is transferred to the final stellated morphologies. Here, general guidelines to stellated nanocrystals are outlined by surveying coreduction of Au and Pd precursors in the presence of a variety of shape-controlled Au seeds to achieve Au/Pd nanostructures. Single-crystalline, twinned, and anisotropic seeds were analyzed to expand the classes of stellated nanostructures synthetically accessible. Significantly, single-crystalline Au seeds adopt {100}-terminated intermediates prior to branching, regardless of initial seed shape. We compared these results with those obtained with shape-controlled Pd seeds, and seed composition was identified as an important synthetic parameter, with Pd seeds being more resistant to shape changes during overgrowth. This difference is attributed to the greater diffusion rate of Au atoms on Au seeds compared to Au atoms on Pd seeds. These results provide guidelines for the seeded synthesis of symmetrically branched nanocrystals and architecturally defined bimetallic nanostructures in general.

Shaping the synthesis and assembly of symmetrically stellated Au/Pd nanocrystals with aromatic additives

Au/Pd octopods were synthesized with enhanced sample homogeneity through the use of aromatic additives. This increase in sample monodispersity facilitates large-area periodic assembly of stellated metal nanostructures for the first time. The aromatic additives were also found to influence the structures of the stellated nanocrystals with subtle shape modifications observed that can alter the packing arrangement of the Au/Pd octopods. These results indicate the possibility of tailored assembly of stellated nanostructures, which would be useful for optical applications that require strong and predictable coupling between plasmonic building blocks.

Surface-enhanced fluorescence immunosensor using Au nano-crosses for the detection of microcystin-LR

A surface-enhanced fluorescence (SEF) immunosensor for the detection of microcystin-LR was developed using Au nano-crosses as fluorescence enhancement nanoparticles and cy5 as a fluorescence label molecule. The SEF effects of cy5 in the proximity of Au nanorods and gold nano-crosses was investigated by using Au nanorods or nano-crosses coated negative-charged glass surfaces. Fluorescence measurements indicated that SEF was influenced by the size, shape and distribution of the Au nanoparticles, with an appropriate spacer layer between the Au nanoparticles and the cy5. The enhancement factor was from 2.3- to 35-fold. Under optimal conditions, the SEF immunosensor exhibited a good linear response at microcystin-LR concentrations of 0.02-16 ng mL(-1) (R(2)=0.9981). The limit of detection was 0.007 ng mL(-1) with little adsorption of microcystin-RR, microcystin-LW, and microcystin-LF. High microcystin-LR recoveries were obtained from naturally contaminated fish samples. The SEF immunosensor allows the reliable detection of microcystin-LR in seafood, and has potential in simple, sensitive detection applications.

High-yield synthesis of triangular gold nanoplates with improved shape uniformity, tunable edge length and thickness

We report the synthesis of uniform triangular gold nanoplates by a modified seeded growth method. The concentration of cetyltrimethylammonium bromide (CTAB) in the growth solution and the time interval between multiple steps of growth were important factors which determined the formation of uniform triangular Au nanoplates. In addition, by further isotropic overgrowth, the thickness of triangular Au nanoplates can be finely tuned within a wide range of 10-80 nm, which at present remains a challenge using conventional seeded growth.

Gold nanorod length controls dispersion, local ordering, and optical absorption in polymer nanocomposite films

The dispersion, local orientation and optical absorption of polystyrene (PS, degree of polymerization P) nanocomposites containing PS-grafted gold nanorods (Au NRs, PS degree of polymerization N), with aspect ratios (ν = length/diameter) ranging from 2.5 to 6.3, are studied using quantitative scanning electron microscopy (SEM) and optical spectroscopy. The experimentally observed nanorod assemblies and optical absorptions are compared with predictions from self-consistent field theory (SCFT) and finite difference time domain (FDTD) calculations, respectively. A pair correlation function for Au NRs is calculated from SEM images, and contains no correlation peaks for P/N = 0.9, indicating nanorods are dispersed within the nanocomposite. Large correlation peaks are observed for P/N = 7.6, representative of interparticle separation distances within nanorod aggregates, which do not vary with ν. On the basis of SCFT calculations, aggregation is attributed to significant depletion-attraction forces in the composite for P/N > 1. When Au NRs disperse, the longitudinal surface plasmon resonance (LSPR) peak red shifts from the visible into the near-IR as ν increases. No shift in the dispersed LSPR position is observed for v = 2.5 and 3.3 upon aggregation because the ratio of the interparticle distance to the nanorod length is too large for surface plasmon coupling. However, for v = 6.3, significant coupling between surface plasmons leads to a blue shift of the LSPR by approximately 140 nm, in agreement with FDTD calculations.

Assembled plasmonic asymmetric heterodimers with tailorable chiroptical response

Directed nanocrystal (NC) heteroassemblies could potentially achieve tailorable multiplex circular dichroism (CD) bands. Here, for the first time, we developed assembly of nanoparticle (NP)-nanorod (NR) chiral heterodimers with chiral molecules to explore their chiroptical activities. The experimental results revealed that plasmonic CD responses were in the region from 520 to 750 nm, which was in agreement with the theoretical simulation. Importantly, the CD band could be regulated by controlling the gaps between adjacent NCs and altering the building blocks of the assemblies. These results show that the plasmonic chiroptical response of NP-NR heterodimers could come from the finger-crossed chiral construction of adjacent NC in the heterodimers and the formation of plasmonic hot-spots in the assemblies could further enhance the plasmonic CD. This work provides a new opportunity to create heterogeneous nanoscale plasmonic objects with tailorable chiroptical response for application in biosensors, in vivo chiral medical carriers and negative refractive index materials.

Photo-Thermal Conversion and Stability of Gold and Silver Nanostructures

Gold and silver nanostructures (such as Au nanorods and Ag nanoplates) exhibit strong and tunable surface plasmon resonance in the near-infrared region (NIR). Under a certain NIR laser irradiation, noble metal nanostructrues achieve a high photo-thermal effect, which would be useful in the therapy. In this work, Au nanorods with longitude surface plasmon resonance (SPRL) shifting in the region of 650 ~1100 nm were synthesized by a seed method. Ag nanoplates and nanocubes with SPR located in the region of 650~850 nm were produced by a hydrothermal method. Through adjusting laser power and irradiating time, the photo-thermal conversions of these nanostructures were studied under NIR laser irradiation. Under low power laser (808 nm, <1W) irradiation, the shape of the Au nanorods are stable and the temperature of colloid increase from room temperature to ~57°C. However, Au nanorods undergo deformation from rod to spherical particle under irradiation of high power (808 nm laser; 6W; 1064nm laser, 7W), resulting in the disappearance of SPRL. Morphology evolutions and photo-thermal conversion of Ag nanostructures were also studied. Ag nanostructures have a lower photo-thermal conversion compared with that of Au nanorods colloid. Snipping and dendrite can be observed for Ag nanoplates after irradiating, while Ag nanocubes have no obvious shape change.

One-pot synthesis of gold nanorods using binary surfactant systems with improved monodispersity, dimensional tunability and plasmon resonance scattering properties

A facile seedless growth method for high-yield synthesis of monodisperse gold nanorods using binary surfactant mixtures is reported for the first time. In comparison with other seedless methods, the present method enables the preparation of gold nanorods with much better monodispersity. Moreover, the present seedless growth method enables the preparation of not only thin gold nanorods but also thick gold nanorods which cannot be prepared by other reported seedless methods. Dark-field microscopy measurements of a single gold nanorod indicate that the thicker gold nanorod shows enhanced scattering properties.

Seeded growth of metal-doped plasmonic oxide heterodimer nanocrystals and their chemical transformation

We have developed a generalized seeded-growth methodology for the synthesis of monodisperse metal-doped plasmonic oxide heterodimer nanocrystals (NCs) with a near-unity morphological yield. Using indium-doped cadmium oxide (ICO) as an example, we show that a wide variety of preformed metal NCs (Au, Pt, Pd, FePt, etc.) can serve as the seeds for the tailored synthesis of metal-ICO heterodimers with exquisite size, shape, and composition control, facilitated by the delayed nucleation mechanism of the CdO phase. The metal-ICO heterodimers exhibit broadly tunable near-infrared localized surface plasmon resonances, and dual plasmonic bands are observed for Au-ICO heterodimers. We further demonstrate that the oxide domain of the Au-ICO heterodimers can be selectively and controllably transformed into a series of partially and completely hollow cadmium chalcogenide nanoarchitectures with unprecedented structural complexity, leaving the metal domain intact. Our work not only represents an exciting addition to the rapidly expanding library of chemical reactions that produce colloidal hybrid NCs, but it also provides a general route for the bottom-up chemical design of multicomponent metal-oxide-semiconductor NCs in a rational and sequential manner.

Drug loaded multilayered gold nanorods for combined photothermal and chemotherapy

In this study, gold nanorods (AuNRs) were first stabilized by hexadecyltrimethylammonium bromide (CTAB) and then coated with two kinds of polyelectrolytes (PE) and BSA to obtain multi-layered AuNRs (AuNRs-PE-BSA). Furthermore, the anti-cancer drug doxorubicin (DOX) was encapsulated into AuNRs-PE-BSA by the electrostatic force and the nanocomposites formed were named AuNRs/DOX-PE-BSA. The success of coating was verified by transmission electron microscopy (TEM), zeta potential, gel-electrophoresis and thermogravimetric analysis (TGA). The MTT assay indicated that the cytotoxicity of AuNRs decreased dramatically after multi-layer capping. The time-dependent nucleus-targeting capability of AuNRs/DOX-PE-BSA was confirmed in cell affinity evaluations. The in vitro and in vivo experiments demonstrated that AuNRs/DOX-PE-BSA, which combined photothermal and chemotherapy for tumor therapy, bears a markedly improved curative effect and holds promising prospects in the field of nanomedicine.

Combining atomistic simulation and X-ray diffraction for the characterization of nanostructures: a case study on fivefold twinned nanowires

Recent progress in achieving high degrees of monodispersity in chemical synthesis of complex nanostructures creates the unique situation in which individual nanostructures become representative for the whole ensemble. Under these conditions, atomistic simulations can play a completely new role in interpreting structural data obtained from averaging techniques. We apply this approach to fivefold twinned Ag nanowires for which the existence of an ambient-stable tetragonal phase in the nanowire core has been recently proposed. Quantitative comparison of experimental X-ray diffraction data with atomistic calculations unequivocally shows that the diffractograms can be fully explained by the complex strain state and defect structure of fivefold twinned Ag nanowires with fcc crystal structure. In addition, our approach enables rapid and accurate determination of wire diameters by a modified Scherrer analysis which uses a database generated by atomistic simulations.

Rapid optimization of metal nanoparticle surface modification with high-throughput gel electrophoresis

The ability to effectively control and optimize surface modification of metal nanoparticles is paramount to the ability to employ metal nanoparticles as diagnostic and therapeutic agents in biology and medicine. Here we present a high-throughput two-dimensional-grid gel electrophoresis cell (2D-GEC)-based method, capable of optimizing the surface modification of as many as 96 samples of metal nanoparticles in approximately 1 h. The 2D-GEC method determines not only the average zeta-potential of the modified particles but also the homogeneity of the surface modification by measuring the distance between the front of the sample track and the area where the maximum optical density is achieved. The method was tested for optimizing pH and concentration of the modifiers (pM) for functionalizing gold nanorod thiol-containing acidic agents.

Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications

The main challenge for all electrical, mechanical and optical sensors is to detect low molecular weight (less than 400 Da) chemical and biological analytes under extremely dilute conditions. Surface plasmon resonance sensors are the most commonly used optical sensors due to their unique ability for real-time monitoring the molecular binding events. However, their sensitivities are insufficient to detect trace amounts of small molecular weight molecules such as cancer biomarkers, hormones, antibiotics, insecticides, and explosive materials which are respectively important for early-stage disease diagnosis, food quality control, environmental monitoring, and homeland security protection. With the rapid development of nanotechnology in the past few years, nanomaterials-enhanced surface plasmon resonance sensors have been developed and used as effective tools to sense hard-to-detect molecules within the concentration range between pmol and amol. In this review article, we reviewed and discussed the latest trend and challenges in engineering and applications of nanomaterials-enhanced surface plasmon resonance sensors (e.g., metallic nanoparticles, magnetic nanoparticles, carbon-based nanomaterials, latex nanoparticles and liposome nanoparticles) for detecting "hard-to-identify" biological and chemical analytes. Such information will be viable in terms of providing a useful platform for designing future ultrasensitive plasmonic nanosensors.

Self-assembly of gold nanorods into vertically aligned, rectangular microplates with a supercrystalline structure

Vertically aligned, supercrystalline microplates with a well-defined rectangular shape were fabricated in a large area through self-assembly of gold nanorods by a novel bulk solution evaporation method. This evaporative self-assembly strategy involving continuous movement of the contact line can prevent the coffee-ring effect, thus allowing uniform deposition of discrete GNR superstructures over a large area and favoring the formation of GNR supercrystals with geometrically symmetric shapes. A mechanism based on the continuing nucleation and growth of smectic GNR superstructures accompanying the movement of the contact line was put forward for the formation of the unique GNR supercrystal arrays. Based on this mechanism, a micropatterned substrate was designed to control the nucleation location and growth direction, leading to the spontaneous self-assembly of nearly parallel arrays of vertically aligned, supercrystalline microplates of GNRs. The obtained rectangular-plate-shaped GNR supercrystals exhibited interesting anisotropic optical reflection properties, which were revealed by polarized light microscopy.

Facile synthesis of silver and bimetallic silver–gold nanoparticles and their applications in surface-enhanced Raman scattering

Monometallic and bimetallic core–shell colloids reported here demonstrate significant SERS signals for a hydrophobic dye molecule at as low as 10 nM and AEF lies within the highest literature values.

Undecylprodigiosin conjugated monodisperse gold nanoparticles efficiently cause apoptosis in colon cancer cells in vitro

Bacterial pigment undecylprodigiosin was conjugated to monodisperse gold nanoparticles, resulting in improved stability and cytotoxicity against colon cancer cells.

Synthesis and properties of quaternary ammonium surfactants containing a methoxy benzyl substitute

m-MDRA-n exhibit high surface activity, excellent adsorptive and bacterial properties, thermodynamic functions of micellization for m-MDRA-n were researched.

Local heating with lithographically fabricated plasmonic titanium nitride nanoparticles

Titanium nitride is considered a promising alternative plasmonic material and is known to exhibit localized surface plasmon resonances within the near-infrared biological transparency window. Here, local heating efficiencies of disk-shaped nanoparticles made of titanium nitride and gold are compared in the visible and near-infrared regions numerically and experimentally with samples fabricated using e-beam lithography. Results show that plasmonic titanium nitride nanodisks are efficient local heat sources and outperform gold nanodisks in the biological transparency window, dispensing the need for complex particle geometries.

Large-Scale Synthesis of Gold Nanorods through Continuous Secondary Growth

Gold nanorods (GNRs) exhibit a tunable longitudinal surface plasmon resonance (LSPR) that depends on the GNR aspect ratio (AR). Independently controlling the AR and size of GNRs remains challenging but is important because the scattering intensity strongly depends on the GNR size. Here, we report a secondary (seeded) growth procedure, wherein continuous addition of ascorbic acid (AA) to a stirring solution of GNRs, stabilized by cetyltrimethylammonium bromide (CTAB) and synthesized by a common GNR growth procedure, deposits the remaining (~70%) of the Au precursor onto the GNRs. The growth phase of GNR synthesis is often performed without stirring, since stirring has been believed to reduce the yield of rod-shaped nanoparticles, but we report that stirring coupled with continuous addition of AA during secondary growth allows improved control over the AR and size of GNRs. After a common primary GNR growth procedure, the LSPR of GNRs is ~820 nm, which can be tuned between ~700-880 nm during secondary growth by adjusting the rate of AA addition or adding benzyldimethylhexadecylammonium chloride hydrate (BDAC). This approach for secondary growth can also be used with primary GNRs of different ARs to achieve different LSPRs and can likely be extended to nanoparticles of different shapes and other metals.

Continuous mesoporous titania nanocrystals: their growth in confined space and scope for application

Enjoying the single lifestyle: With an overwhelming efficiency compared to thermally sintered preformed nanocrystals, mesoporous single crystals (MSCs) of TiO2 constitute a new class of semiconductor materials for low-cost solar power, solar fuel, photocatalysis, and energy storage applications. This Highlight explores the benefits of template-directed seed-mediated growth in the confined space of a preseeded mesoporous template, and possible research avenues for further improvements.

Large scale solution assembly of quantum dot-gold nanorod architectures with plasmon enhanced fluorescence

Tailoring the efficiency of fluorescent emission via plasmon-exciton coupling requires structure control on a nanometer length scale using a high-yield fabrication route not achievable with current lithographic techniques. These systems can be fabricated using a bottom-up approach if problems of colloidal stability and low yield can be addressed. We report progress on this pathway with the assembly of quantum dots (emitter) on gold nanorods (plasmonic units) with precisely controlled spacing, quantum dot/nanorod ratio, and long-term colloidal stability, which enables the purification and encapsulation of the assembled architecture in a protective silica shell. Overall, such controllability with nanometer precision allows one to synthesize stable, complex architectures at large volume in a rational and controllable manner. The assembled architectures demonstrate photoluminescent enhancement (5×) useful for applications ranging from biological sensing to advanced optical communication.

Shape alloys of nanorods and nanospheres from self-assembly

Mixtures of anisotropic nanocrystals promise a great diversity of superlattices and phase behaviors beyond those of single-component systems. However, obtaining a colloidal shape alloy in which two different shapes are thermodynamically coassembled into a crystalline superlattice has remained a challenge. Here we present a joint experimental-computational investigation of two geometrically ubiquitous nanocrystalline building blocks-nanorods and nanospheres-that overcome their natural entropic tendency toward macroscopic phase separation and coassemble into three intriguing phases over centimeter scales, including an AB2-type binary superlattice. Monte Carlo simulations reveal that, although this shape alloy is entropically stable at high packing fraction, demixing is favored at experimental densities. Simulations with short-ranged attractive interactions demonstrate that the alloy is stabilized by interactions induced by ligand stabilizers and/or depletion effects. An asymmetry in the relative interaction strength between rods and spheres improves the robustness of the self-assembly process.

Tunable loading of single-stranded DNA on gold nanorods through the displacement of polyvinylpyrrolidone

A quantitative and tunable loading of single-stranded (ss-DNA) molecules onto gold nanorods was achieved through a new method of surfactant exchange. This new method involves the exchange of cetyltrimethylammonium bromide surfactants for an intermediate stabilizing layer of polyvinylpyrrolidone and sodium dodecylsulfate. The intermediate layer of surfactants on the anisotropic gold particles was easily displaced by thiolated ss-DNA, forming a tunable density of single-stranded DNA molecules on the surfaces of the gold nanorods. The success of this ligand exchange process was monitored in part through the combination of extinction, X-ray photoelectron, and infrared absorption spectroscopies. The number of ss-DNA molecules per nanorod for nanorods with a high density of ss-DNA molecules was quantified through a combination of fluorescence measurements and elemental analysis, and the functionality of the nanorods capped with dense monolayers of DNA was assessed using a hybridization assay. Core-satellite assemblies were successfully prepared from spherical particles containing a probe DNA molecule and a nanorod core capped with complementary ss-DNA molecules. The methods demonstrated herein for quantitatively fine tuning and maximizing, or otherwise optimizing, the loading of ss-DNA in monolayers on gold nanorods could be a useful methodology for decorating gold nanoparticles with multiple types of biofunctional molecules.

Gold nanorods and their plasmonic properties

Gold nanorods have been receiving extensive attention owing to their extremely attractive applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices. The growth methods and plasmonic properties of Au nanorods have therefore been intensively studied. In this review, we present a comprehensive overview of the flourishing field of Au nanorods in the past five years. We will focus mainly on the approaches for the growth, shape and size tuning, functionalization, and assembly of Au nanorods, as well as the methods for the preparation of their hybrid structures. The plasmonic properties and the associated applications of Au nanorods will also be discussed in detail.

Seed-mediated growth of ultralong gold nanorods and nanowires with a wide range of length tunability

This study reports a systematic approach to synthesize ultralong gold nanorods and nanowires using a seed-mediated growth approach. In the first series, the effect of growth solution pH on the lengths of nanorods prepared was investigated. Interestingly, although shorter rods (230-310 nm) were produced in a basic solution environment than in an acidic condition (330-410 nm), the nanorod yield is greatly improved with relatively few nanoplate byproducts formed. Nanorod growth proceeds quickly in a basic solution as evidenced by the fast solution color changes. By adjusting several experimental parameters with the aim to elongate the nanorod length in a tunable fashion, gold nanorods and nanowires with average lengths from 580 to 2850 nm can be synthesized by progressively increasing the HNO3 concentration in the final growth solution. Nanowire growth in a highly acidic solution is slower, and a substantially longer time is needed to reach long lengths. Further extension of the nanowire length can be achieved simply by reducing the volume of second growth solution transferred to the final growth solution. Nanorods and nanowires with lengths spanning from 700 nm to 4.5 μm were prepared in this series of experimental conditions. The longest nanowires can reach a length of up to 6 μm. The nanowires still maintain thin average diameters of 33-53 nm. The ability to make gold nanorods and nanowires over this exceptionally wide and useful length range is exciting because applications and demonstrations using ultralong gold nanorods and nanowires of most suitable lengths are now possible.

Size-controlled synthesis of gold bipyramids using an aqueous mixture of CTAC and salicylate anions as the soft template

One-dimensional (1D) gold (Au) bipyramids are successfully synthesized through a facile seed-mediated method using cetyltrimethylammonium chloride (CTAC), Au seed nanoparticles, Ag(+) ions, and ascorbic acid. The length and optical properties of the synthesized Au bipyramids are controlled with precision by varying the amount of salicylate anions (Sal(-)) added during the synthesis. The micelles formed from CTA(+)-Sal(-) mixtures in aqueous solutions act as effective templates for the size-controlled synthesis of 1D nanocrystals.

Core-shell noble metal nanostructures templated by gold nanorods

The main research progress in core-shell noble metal nanostructures templated by gold nanorods (Au NRs) is summarized regarding synthesis, optical, and catalytic properties. Design and fabrication of core-shell hybrid nanostructures are demonstrated to be effective not only for optimizing and expanding intrinsic properties but also for creating novel localized surface plasmon enhanced optical and catalytic functionalities, thus providing great prospects in both fundamental research and potential applications.

Packing and self-assembly of truncated triangular bipyramids

Motivated by breakthroughs in the synthesis of faceted nano- and colloidal particles, as well as theoretical and computational studies of their packings, we investigate a family of truncated triangular bipyramids. We report dense periodic packings with small unit cells that were obtained via numerical and analytical optimization. The maximal packing fraction φ(max) changes continuously with the truncation parameter t. Eight distinct packings are identified based on discontinuities in the first and second derivatives of φ(max)(t). These packings differ in the number of particles in the fundamental domain (unit cell) and the type of contacts between the particles. In particular, we report two packings with four particles in the unit cell for which both φ(max)(t) and φ(max)'(t) are continuous and the discontinuity occurs in the second derivative only. In the self-assembly simulations that we perform for larger boxes with 2048 particles, only one out of eight packings is found to assemble. In addition, the degenerate quasicrystal reported previously for triangular bipyramids without truncation [Haji-Akbari et al., Phys. Rev. Lett. 107, 215702 (2011)] assembles for truncations as high as 0.45. The self-assembly propensities for the structures formed in the thermodynamic limit are explained using the isoperimetric quotient of the particles and the coordination number in the disordered fluid and in the assembled structure.